Filmmaker Nick Broomfield’s latest documentary, a portrait of former Republican vice-presidential candidate Sarah Palin, received its UK premiered on Friday as part of the BFI London Film Festival.

Speaking to a packed crowd after the screening, the 63-year-old BAFTA winner said that Sarah Palin You Betcha! was one of his most difficult documentaries to date as he was forced to make a film about his subject “from afar”.

Broomfield spends the first part of the film cosying up to Palin’s family and former friends. He even meets the subject at a book signing. His request for an interview is met with a “you betcha!”

Dressed like a lumberjack and forever slipping on the Alaskan ice, Broomfield cuts an eccentric figure, but one that seems to be making headway towards his subject.

Yet this makes the second half all the more sinister as the small Wasilla community shuts down, becoming increasingly reluctant to take part.

“There was group of people who did agree to talk to us, but anyone that went to school with her, or who had grown up with her, or whose friends were friends with her kids, they had to carry on living in Wasilla so they were reluctant,” he said.

“The people who did talk did so because they thought they ought to. They thought she was a menace, but I’m sure these people will have a hard time in Wasilla when the film comes out.”

The film, which begins with Palin’s acceptance speech after being selected as McCain’s running-mate for the 2008 election, is shot almost entirely on location in Palin’s home town, a place where there are “8,000 people, 27 churches and a lot of superstores”.

It charts her life from teenage basketball player through school to mayor, governor and finally national politician, concluding that there is in fact two people – the public Palin and the private one.

Even according to the interviewees who, with the exception of Palin’s parents, were near-universally critical, the public Palin is a “charismatic” woman who “could make you feel like you were the only person in the room”.

Yet the real vitriol was reserved for the private Palin, who at best was painted as an uneducated, small-minded, small town, text-message addict who struck it lucky in politics.

At worst, she comes across as a “dangerous and frightening” sociopath; a woman, as one Wasilla resident describes her, who “wouldn’t think twice about killing you if you got in her way”.

Readers of the recent biography by Geoffrey Dunn, the hugely critical The Lies of Sarah Palin: The Untold Story Behind Her Relentless Quest for Power, will find scant new revelations in the film. However, by spending time in Wasilla and speaking to residents on camera, Broomfield manages to create a convincing illustration of the forces – family, religion, upbringing – that went into Palin’s makeup, pulling in footage from his subject’s early years, trying her hand as a newscaster and participating in a beauty contest.

In one archive clip, the former mayor is seen pardoning a Christmas turkey only to give an interview to local TV minutes later in which the bird is visible in the background having its head removed.

Despite several moments of levity, usually provided by Broomfield, the tone of the piece remains earnest. This is, after all, the woman who placed crosshairs targeting Democratic states just weeks before the Tucson shootings.

This is also the woman that, according to the film, campaigned for mayor as a “Christian” whilst suggesting that the incumbent, her former mentor John Stein, was Jewish.

Palin’s apocalyptic faith is a theme throughout. As one of her former friends says: “She would have no conscience about triggering a nuclear war. She believes she is God’s anointed one. If you don’t know that, you don’t know anything about Sarah Palin.”

For Broomfield, Palin’s recent refusal to run for president in 2012 probably means the end of her political career. “She’s done,” he said to an audience member after the screening.

However, the popularity she still enjoys with great swathes of the American electorate has opened the door for the more extreme elements currently vying for Republican endorsement.

“The evangelical right are a massive force in the Republican Party and they’ve become more so,” said Broomfield.

“No one is really in control of them and they [the evangelical right] have had a massive effect on politics, especially when you see people like Palin and Michele Bachmann, who are a manifestation of that movement.

“They’ve moved the whole thing to the right. Until the union between Wall Street and the evangelicals is broken, I think US politics is going to be quite grim and depressing. It’s a bad time in American history and I think she [Palin], more than anything, embodies that.”

Broomfield admitted that the more he learned about Palin, the more he found her “disquieting”,

“I felt like there was always another secret about her, or another way of explaining her. She changed her positions politically very often. She would always just go where the power is… so she’s ended up in the extreme right with the Koch brothers and Murdoch by supporting lower taxation a deregulation… arguing that not taxing the corporations would bring about higher employment, which is just crazy stuff.

“But at another time in her career, such as when she was governor of Alaska, she put a massive tax on the oil companies, which is entirely contrary to what the Republicans believe in… and she did that with Democrat support. So she’s been wherever she can wrestle power. It’s just very hard to pin her down.”

The most remarkable scenes from the film remain the interviews from her 2008 run. Played in montage, it seems incomprehensible that the McCain team picked a candidate so clearly inappropriate for the job of vice president. Yet they did and against a lesser campaigner than Obama, they may well have won.

It would be easy to write Palin off as a quirk or a foible of history. But the fact that she made it so close to the White House should give everyone genuine pause, and particularly those looking on at the current race for the Republican nomination.

What many will take from the film is that in American politics anything is possible. Does that mean Perry, Bachmann and the rest of their ilk have a genuine shot at the top job? If the experience of Mama Grizzly has taught us anything, the answer has to be “you betcha!”